In general, braking force was produced using four hydraulic brakes in a brake system for a vehicle, but recently, usage of an electronic brake tends to gradually increase because of development of the electronic brake.
The hydraulic brake and the electronic brake each have advantages and disadvantages. For example, in a case in which the electronic brake is applied to all vehicle wheels, electric power consumption, which is accompanied with an operation of the electronic brake, is increased, and braking force cannot absolutely be obtained in the case of failure of the electric system. Therefore, a method of using the electronic brake for front wheels, and using the hydraulic brake for rear wheels may be considered.
FIG. 1 is a view illustrating a configuration of a combined brake system.
Referring to FIG. 1, a combined brake system 10 has a configuration in which a brake operation for the front wheels is performed by EMBs 13 and by an electric brake apparatus, and braking force for the rear wheels is produced through hydraulic brake calipers 14 by directly transmitting hydraulic pressure without using a booster.
A pedal unit 11 determines brake intention of a driver through an angle sensor or a stroke sensor, and sends the brake intention to an ECU 12 (controller). Without using the booster, braking force generated by the driver is boosted only by a multiple of a difference in cross-sectional area between a master cylinder and a wheel cylinder of the caliper, and directly transmitted to hydraulic calipers 14 for the rear wheels. The aforementioned configuration that has no booster is possible because rear wheel braking force is smaller than front wheel braking force in terms of required braking force.
The ECU 12, which receives the brake intention, controls the front wheel EMB 13 so as to generate the front wheel braking force.
However, in the system 10 in which the electronic brake and the hydraulic brake are used together as described above, a configuration, which includes all constituent elements in the related art is difficult to configure a hydraulic brake apparatus.
Particularly, because a pedal feel is provided only by a rear wheel cylinder (hydraulic manner) having a small required liquid amount, there is a problem in that it is difficult to produce a normal pedal feel that is similar to a pedal feel in a system vehicle that uses a hydraulic brake system in the related art or a pedal simulator. A drum brake is more disadvantageous than a disk brake in terms of the required liquid amount.
FIG. 2 is a graph illustrating a relationship between pedal effort and a pedal stroke of a driver in a general hydraulic brake system.
The graph of FIG. 2 compares different pedal feels in accordance with types of vehicles, and although there are differences in extents, the pedal feels each commonly are divided into an ineffective stroke section, a primary pedal effort section A, a secondary pedal effort section B, and a pressure reduction section C.
Here, an important point is a difference in gradient between the primary pedal effort section A and the secondary pedal effort section B, and the difference in gradient occurs because of a boost limit (full load point) of the booster of the hydraulic brake apparatus. The gradient of the primary pedal effort section A is gentle because this section is boosted by the booster, and the gradient of the secondary pedal effort section B is rapidly increased because the boosting operation of the booster ends in this section.
However, in the combined brake system in the related art, only the primary pedal effort section is shown in a case in which the braking force is produced by simply connecting the existing master cylinder to a rear wheel hydraulic brake apparatus, and as a result, there is a problem in that the driver may feel heterogeneity regarding a brake operation. That is, there should be a section in which the pedal effort is rapidly increased in a section having a predetermined stroke or more, but otherwise the driver may feel inconvenience.